Gas separation processes with membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The design of new materials and efficient methods will further advance the membrane gas separation processes within the next decade.
The gas transport properties of many glassy and rubbery polymers have been measured, driven by the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, an important limitation in the development of new membranes for gas separation applications is the well-known trade-off between permeability and selectivity, as first shown by Robeson. See Robeson, J. MEMBR. SCI., 62: 165 (1991); Robeson, CURR. OPIN. SOLID STATE MATER. SCI., 4: 549 (1999). By comparing the data of hundreds of different polymers, he demonstrated that selectivity and permeability seem to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.
Despite concentrated efforts to tailor polymer structure to affect separation properties, current polymeric membrane materials have seemingly reached a limit in the tradeoff between productivity and selectivity. See Zimmerman, et al., J. MEMBR. SCI., 137: 145 (1997). For example, many polyimide and polyetherimide glassy polymers such as Ultem 1000 have much higher intrinsic CO2/CH4 selectivities (˜30 at 50° C. and 100 psig) than that of cellulose acetate (CA, ˜22), which are more attractive for practical gas separation applications. These polymers, however, do not have outstanding permeabilities attractive for commercialization compared to current UOP Separex CA membrane product, completely in agreement with the Robeson trade-off relation.
Our previous study has shown that nano-molecular sieves such as poly(ethylene glycol) (PEG)-functionalized nano-Silicalite or nano-SAPO-34 dispersed in CA-based mixed matrix membranes (MMM) can enhance the CO2 permeability over the intrinsic CO2 permeability of the pure CA polymer matrix, and in the meantime the CO2/CH4 selectivity (αCO2/CH4) remained almost the same as that of CA polymer matrix. The αCO2/CH4 of nano-molecular sieve-CA MMM films (<22), however, is still not high enough for the next generation of UOP Separex membrane product for CO2 removal from natural gas.
Therefore, the aim of the present invention is to prepare nano-molecular sieve-polymer MMM membranes to achieve higher αCO2/CH4 than that of CA membrane with at least higher than 5 barrer CO2 permeability, which is promising for practical application. We studied the use of template-free nano-molecular sieves, such as template-free nano-Silicalite, nano-AlPO-18, nano-SAPO-34, and PEG-functionalized nano-Silicalite, as the dispersed phase in MMM films using Matrimid 5218 and Ultem 1000 continuous polymer matrices. Experimental pure gas permeation results demonstrated significantly improved CO2/CH4 separation properties.